The three of them decided to investigate the mysterious disappearances, and John knew the first place they should go. "My brother and I," John said, "have been tracking the disappearances. You know, who disappears, where, what they were wearing, who saw them last, and we've been trying to fit the puzzle together. Find out if there's a connection between any of the victims and what it is."
"So," Clara said, "standard nutters, then?"
"Exactly," John replied.
As they walked to John and his brother's house, the Doctor had a sense of foreboding. Nothing powerful or hugely ominous, just a little nagging feeling in the back of his head that something wasn't quite right. His suspicions were confirmed when they arrived at the house. The door was open inward, obviously due to a great impact. It was completely off the top hinge and just barely hanging on by the lower one. It was a reflection of the state of the rest of the house. As the trio entered the home, it was obvious that every single item in the home, from huge stacks of papers to an assortment of strange looking gadgets and pieces of equipment, used to be meticulously organized. However, now, most of the paper was strewn about the floor, and much of the equipment had endured blows from someone who clearly wanted the work destroyed. Clara let out a gasp at the first sight of the destruction. John just stared for a while, taking in the detritus that he had worked so hard on. For a while, nobody spoke.
"I don't know what to say," said the Doctor.
"It's- it's fine," said John. "I knew this would happen eventually."
"Let's just find whoever did this and put an end to all this," Clara said.
"I agree," John said. "There's just one problem. The key to finding them was supposed to be here, but it's missing."
"Yeah?" Clara said. "What is it?"
"It's my brother," John said. "He's been taken."
As they walked down the street, away from the ruins of John's house, it was clear that John was in great distress, but trying to hide it with overconfidence. He was failing. "What are we going to do, Doctor?" he said. "We have no leads to go on, and we don't even know where to start looking!" Just as he said this, they heard screams and sounds of conflict coming from not far from where they were.
"What's that?" Clara asked. The corner of the Doctor's lip turned up in a smile.
"That would be our lead!" he said before taking off at a run towards the source of the ruckus, with Clara and John right behind him.
They arrived on the scene just in time. A figure was running away from the ransacked house. The trio gave chase to the figure, who turned a corner once it reached the end of the street. They followed suit, continuing to pursue the figure, even after it had run out of the city gates. And still the figure continued to run, across the snowy field and toward the base of a nearby mountain. It was plain to see where he was going: a cave that made its way inside the mountain. They reached the mouth of the cave, and something odd happened. The running figure faded out of existence, leaving the Doctor, Clara, and John alone in the cave, which was filled with various screens, instruments, and gadgets. While the Doctor and Clara were still a bit disoriented from the chase, John performed yet another unexpected action: he pressed a button on a nearby control panel, causing a door to slide down over the cave entrance.
They were all locked inside.